Review on Stack Overflow Survey Results

Problem Statement

Data is taken from Stack Overflow Annual Developer Survey (2018-2021). The survey data will reviewed in two aspects:

To compare income of developers & data experts across countries, and factoring in the effect of tax and purchasing power.
To evaluate the effect of years of professional coding experience on their income, after adjusting for usage of Python programming and education level.

For our data science project, we activated the following packages. We may directly call functions in other packages with :: to avoid namespace collisions, given that data used in this project is not huge, there should be no concerns about the efficiency.

# Load necessary packages
pacman::p_load(tidyverse, lubridate, modelr, broom, plotly, CGPfunctions, equatiomatic, rvest ) 

# Setting working directory, seed & options
setwd('C:/SMU Study/R Programming/DASR/Capstone Project/Submission/')
set.seed(20211031)
options(scipen = 9999)
defaultW <- getOption("warn")
options(warn = -1)
options(tibble.width = Inf) # displays all columns.
options(tibble.print_max = Inf) # to show all the rows.
options(dplyr.summarise.inform = FALSE)  # Suppress summarise info

Import

We imported the results of survey conducted from 2018 to 2021.

# data18=read_csv("C:/SMU Study/R Programming/DASR/Capstone Project/Data/Data Scientist Earn/2018/survey_results_public.csv")
# data19=read_csv("C:/SMU Study/R Programming/DASR/Capstone Project/Data/Data Scientist Earn/2019/survey_results_public.csv")
# data20=read_csv("C:/SMU Study/R Programming/DASR/Capstone Project/Data/Data Scientist Earn/2020/survey_results_public.csv")
# data21=read_csv("C:/SMU Study/R Programming/DASR/Capstone Project/Data/Data Scientist Earn/2021/survey_results_public.csv")

# Save raw dataset files in .RDS format
# saveRDS(data18, file = "LEE_data18.rds")
# saveRDS(data19, file = "LEE_data19.rds")
# saveRDS(data20, file = "LEE_data20.rds")
# saveRDS(data21, file = "LEE_data21.rds")

data18 <- readRDS("LEE_data18.rds")
data19 <- readRDS("LEE_data19.rds")
data20 <- readRDS("LEE_data20.rds")
data21 <- readRDS("LEE_data21.rds")

Data categories & variable names used in each survey year may be slightly different, we will align the values and rename the variables.

Within the datasets, there are Converted Annual Compensation in US dollar (to be named as ConvertedCompYearly), which will serve as our dependent (continuous y) variable.

Also, the dataset contains various attributes of the respondents, such as Country, Years of Professional Coding (YearsCodingProf), Gender, Education Level (EdLevel) etc.

In Problem 1, Country will be our dependent variable (categorical x1), and it will show the significant difference of income among countries.

Hence in Problem 2, we will only focus on the country of United Stated which has the most respondents in this survey, YearsCodingProf will serve as our dependent variable (continuous x1); usage of Python programming (fPython, derived from LanguageWorkedWith) and EdLevel will be one by one applied as our moderator (categorical x2).

# glimpse(data18)
# glimpse(data19)
# glimpse(data20)
glimpse(data21)

## Rows: 83,439
## Columns: 48
## $ ResponseId                   <dbl> 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13~
## $ MainBranch                   <chr> "I am a developer by profession", "I am a~
## $ Employment                   <chr> "Independent contractor, freelancer, or s~
## $ Country                      <chr> "Slovakia", "Netherlands", "Russian Feder~
## $ US_State                     <chr> NA, NA, NA, NA, NA, "Georgia", "New Hamps~
## $ UK_Country                   <chr> NA, NA, NA, NA, "England", NA, NA, NA, NA~
## $ EdLevel                      <chr> "Secondary school (e.g. American high sch~
## $ Age1stCode                   <chr> "18 - 24 years", "11 - 17 years", "11 - 1~
## $ LearnCode                    <chr> "Coding Bootcamp;Other online resources (~
## $ YearsCode                    <chr> NA, "7", NA, NA, "17", NA, "3", "4", "6",~
## $ YearsCodePro                 <chr> NA, NA, NA, NA, "10", NA, NA, NA, "4", "4~
## $ DevType                      <chr> "Developer, mobile", NA, NA, "Developer, ~
## $ OrgSize                      <chr> "20 to 99 employees", NA, NA, "100 to 499~
## $ Currency                     <chr> "EUR European Euro", NA, NA, "EUR Europea~
## $ CompTotal                    <dbl> 4800, NA, NA, NA, NA, NA, NA, NA, NA, 420~
## $ CompFreq                     <chr> "Monthly", NA, NA, "Monthly", NA, NA, NA,~
## $ LanguageHaveWorkedWith       <chr> "C++;HTML/CSS;JavaScript;Objective-C;PHP;~
## $ LanguageWantToWorkWith       <chr> "Swift", NA, "Julia;Python;Rust", "JavaSc~
## $ DatabaseHaveWorkedWith       <chr> "PostgreSQL;SQLite", "PostgreSQL", "SQLit~
## $ DatabaseWantToWorkWith       <chr> "SQLite", NA, "SQLite", NA, "Cassandra;El~
## $ PlatformHaveWorkedWith       <chr> NA, NA, "Heroku", NA, NA, NA, NA, "Heroku~
## $ PlatformWantToWorkWith       <chr> NA, NA, NA, NA, NA, NA, NA, "Heroku", NA,~
## $ WebframeHaveWorkedWith       <chr> "Laravel;Symfony", "Angular;Flask;Vue.js"~
## $ WebframeWantToWorkWith       <chr> NA, NA, "Flask", "Angular;jQuery", "Flask~
## $ MiscTechHaveWorkedWith       <chr> NA, "Cordova", "NumPy;Pandas;TensorFlow;T~
## $ MiscTechWantToWorkWith       <chr> NA, NA, "Keras;NumPy;Pandas;TensorFlow;To~
## $ ToolsTechHaveWorkedWith      <chr> NA, "Docker;Git;Yarn", NA, NA, "Docker;Gi~
## $ ToolsTechWantToWorkWith      <chr> NA, "Git", NA, NA, "Docker;Git;Kubernetes~
## $ NEWCollabToolsHaveWorkedWith <chr> "PHPStorm;Xcode", "Android Studio;Intelli~
## $ NEWCollabToolsWantToWorkWith <chr> "Atom;Xcode", NA, "IPython/Jupyter;RStudi~
## $ OpSys                        <chr> "MacOS", "Windows", "MacOS", "Windows", "~
## $ NEWStuck                     <chr> "Call a coworker or friend;Visit Stack Ov~
## $ NEWSOSites                   <chr> "Stack Overflow", "Stack Overflow", "Stac~
## $ SOVisitFreq                  <chr> "Multiple times per day", "Daily or almos~
## $ SOAccount                    <chr> "Yes", "Yes", "Yes", "Yes", "Yes", "Yes",~
## $ SOPartFreq                   <chr> "A few times per month or weekly", "Daily~
## $ SOComm                       <chr> "Yes, definitely", "Yes, definitely", "Ye~
## $ NEWOtherComms                <chr> "No", "No", "Yes", "No", "No", "No", "No"~
## $ Age                          <chr> "25-34 years old", "18-24 years old", "18~
## $ Gender                       <chr> "Man", "Man", "Man", "Man", "Man", "Prefe~
## $ Trans                        <chr> "No", "No", "No", "No", "No", "No", "No",~
## $ Sexuality                    <chr> "Straight / Heterosexual", "Straight / He~
## $ Ethnicity                    <chr> "White or of European descent", "White or~
## $ Accessibility                <chr> "None of the above", "None of the above",~
## $ MentalHealth                 <chr> "None of the above", "None of the above",~
## $ SurveyLength                 <chr> "Appropriate in length", "Appropriate in ~
## $ SurveyEase                   <chr> "Easy", "Easy", "Easy", "Neither easy nor~
## $ ConvertedCompYearly          <dbl> 62268, NA, NA, NA, NA, NA, NA, NA, NA, 51~

1. DATA PRE-PROCESSING

A few points have to be catered in the step:

Some variables with the same meaning but different names across the survey years, they have to be renamed and aligned;
Some variables refer to the same data item but grouped in different categories; e.g. Years of Professional Coding in 2018 is in broad category while after 2018 it is categorized by each year for those below 50 years, they have to be re-grouped or data of 2018 has to be excluded from analysis;
Compensation in USD may be in different frequencies, they will be converted to yearly;
Some variables in some years may not be available, they will be added with default values so that we could append data in all years for further review;
Years of Professional Coding will be one of the independent variable, hence it will be centered by substracting its mean value;

data18a <- data18 %>% 
  mutate(year=2018, 
         ConvertedCompYearly=ifelse(SalaryType=="Yearly", ConvertedSalary,
                                    ifelse(SalaryType=="Monthly", ConvertedSalary*12,
                                           ifelse(SalaryType=="Weekly", ConvertedSalary*52, NA) ) )
         ) %>% 
  mutate(YearsCodingProf=factor(YearsCodingProf, 
                                levels=c("0-2 years", "3-5 years", "6-8 years", "9-11 years",
                                         "12-14 years", "15-17 years", "18-20 years", "21-23 years",
                                         "24-26 years", "27-29 years", "30 or more years"))
         ) %>% 
  mutate(MainBranch="Z.Missing") %>% 
  rename(Hobbyist=Hobby,
         EdLevel=FormalEducation, Ethnicity=RaceEthnicity,
         JobSat=JobSatisfaction, OpSys=OperatingSystem,
         OrgSize=CompanySize) %>% 
  select(-Age)
  
data19a <- data19 %>%
  mutate(year=2019, 
         ConvertedCompYearly=ifelse(CompFreq=="Yearly", ConvertedComp,
                                    ifelse(CompFreq=="Monthly", ConvertedComp*12,
                                           ifelse(CompFreq=="Weekly", ConvertedComp*52, NA) ) )
         ) %>% 
  mutate(YearsCodingProf = ifelse(YearsCodePro=="Less than 1 year", 0,
                                  ifelse(YearsCodePro=="More than 50 years",99,
                                         YearsCodePro) ),
         nCodeExp=as.numeric(YearsCodingProf),
         YearsCodingProf = cut(nCodeExp, breaks=c(-1, seq(2,30, 3) , 99),
                               labels=c("0-2 years", "3-5 years", "6-8 years", "9-11 years",
                                        "12-14 years", "15-17 years", "18-20 years", "21-23 years", 
                                        "24-26 years", "27-29 years", "30 or more years") )
         ) %>% 
  select(-Age)
# levels(data19$YearsCodingProf)

data20a <- data20 %>% 
  mutate(year=2020, 
         ConvertedCompYearly=ifelse(CompFreq=="Yearly", ConvertedComp,
                                    ifelse(CompFreq=="Monthly", ConvertedComp*12,
                                           ifelse(CompFreq=="Weekly", ConvertedComp*52, NA) ) )
         ) %>% 
  mutate(YearsCodingProf = ifelse(YearsCodePro=="Less than 1 year", 0,
                                  ifelse(YearsCodePro=="More than 50 years",99,
                                         YearsCodePro) ),
         nCodeExp=as.numeric(YearsCodingProf),
         YearsCodingProf = cut(nCodeExp, breaks=c(-1, seq(2,30, 3) , 99),
                               labels=c("0-2 years", "3-5 years", "6-8 years", "9-11 years", 
                                        "12-14 years", "15-17 years", "18-20 years", "21-23 years",     
                                        "24-26 years", "27-29 years", "30 or more years") )
         ) %>% 
  mutate(Student="Z.Missing") %>% 
  select(-Age)

data21a <- data21 %>% 
  mutate(year=2021, YearsCodingProf = ifelse(YearsCodePro=="Less than 1 year", 0,
                                             ifelse(YearsCodePro=="More than 50 years",99,
                                                    YearsCodePro) ),
         nCodeExp=as.numeric(YearsCodingProf),
         YearsCodingProf = cut(nCodeExp, breaks=c(-1, seq(2,30, 3) , 99),
                               labels=c("0-2 years", "3-5 years", "6-8 years", "9-11 years", 
                                        "12-14 years", "15-17 years", "18-20 years", "21-23 years",     
                                        "24-26 years", "27-29 years", "30 or more years") )
  ) %>% 
  mutate(CurrencySymbol = substr(Currency, 1, 3) ) %>% 
  mutate(Student="Z.Missing") %>% 
  rename(LanguageWorkedWith=LanguageHaveWorkedWith, 
         DatabaseWorkedWith=DatabaseHaveWorkedWith,
         Respondent=ResponseId) %>% 
  select(-Age)

Top 10 Countries with most Respondents

The first problem is to compare the income across countries, however, if countries do not have sufficient number of respondents, the analysis result would be inconclusive. Hence it is proposed to focus on the top 10 countries with most respondents.

ho <- 
  bind_rows(data18a %>%
              select(ConvertedCompYearly, Country),
            data19a %>%
              select(ConvertedCompYearly, Country),
            data20a %>%
              select(ConvertedCompYearly, Country),
            data21a %>%
              select(ConvertedCompYearly, Country)
            ) %>% 
  filter(!is.na(ConvertedCompYearly) & ConvertedCompYearly>0 ) %>% 
  mutate(Country=ifelse(Country=="United States of America", "United States", Country)) %>% 
  mutate(Country=ifelse(Country=="United Kingdom of Great Britain and Northern Ireland", "United Kingdom", Country)) %>% 
  group_by(Country) %>% 
  summarise(N=n()) %>% 
  ungroup() %>% 
  arrange(desc(N) )
Top10_Country <- ho$Country[1:10]
Top10_Country

##  [1] "United States"  "India"          "United Kingdom" "Germany"       
##  [5] "Canada"         "France"         "Brazil"         "Poland"        
##  [9] "Netherlands"    "Australia"

Identifiy the most common Developer Type, Language & Database

This information will be used to derive indicators in subsequent steps.

hoTemp <- 
  bind_rows(data18a %>%
              select(ConvertedCompYearly, Country, DevType, LanguageWorkedWith, DatabaseWorkedWith),
            data19a %>%
              select(ConvertedCompYearly, Country, DevType, LanguageWorkedWith, DatabaseWorkedWith),
            data20a %>%
              select(ConvertedCompYearly, Country, DevType, LanguageWorkedWith, DatabaseWorkedWith),
            data21a %>%
              select(ConvertedCompYearly, Country, DevType, LanguageWorkedWith, DatabaseWorkedWith)
            ) %>% 
  filter(!is.na(ConvertedCompYearly) & ConvertedCompYearly>0) 

ho <- hoTemp %>%
  select(DevType) %>% 
  separate(col=DevType,
           paste0("v", str_pad( as.character( seq(1,40,1)), 2, pad="0" )  ),
           sep = ";") 
table(ho$v40)  # To ensure all BLANK

## < table of extent 0 >

AllDevType <- ho %>%
  pivot_longer(cols=paste0("v", str_pad( as.character( seq(1,40,1)), 2, pad="0" )  ),
               names_to = "vv",
               values_to = "role") %>% 
  filter(!is.na(role)) %>% 
  group_by(role) %>% 
  summarise(NN=n()) %>% 
  arrange(desc(NN))
head(AllDevType, 25)

## # A tibble: 25 x 2
##    role                                             NN
##    <chr>                                         <int>
##  1 Developer, full-stack                         74524
##  2 Developer, back-end                           69658
##  3 Developer, front-end                          44047
##  4 Developer, desktop or enterprise applications 28142
##  5 Back-end developer                            25008
##  6 Developer, mobile                             22522
##  7 DevOps specialist                             22164
##  8 Full-stack developer                          21701
##  9 Database administrator                        20753
## 10 System administrator                          18676
## 11 Designer                                      16170
## 12 Front-end developer                           15973
## 13 Data scientist or machine learning specialist 12926
## 14 Data or business analyst                      12741
## 15 Developer, embedded applications or devices   11228
## 16 Engineering manager                           10179
## 17 Developer, QA or test                          9975
## 18 Engineer, data                                 9637
## 19 Student                                        9388
## 20 Product manager                                8195
## 21 Mobile developer                               8034
## 22 Desktop or enterprise applications developer   7536
## 23 Academic researcher                            7258
## 24 Educator                                       6116
## 25 Engineer, site reliability                     5454

ho <- hoTemp %>% 
  select(LanguageWorkedWith) %>% 
  separate(col=LanguageWorkedWith,
           paste0("v", str_pad( as.character( seq(1,40,1)), 2, pad="0" )  ),
           sep = ";") 
table(ho$v40)  # To ensure all BLANK

## < table of extent 0 >

AllLang <- ho %>%
  pivot_longer(cols=paste0("v", str_pad( as.character( seq(1,40,1)), 2, pad="0" )  ),
               names_to = "vv",
               values_to = "role") %>% 
  filter(!is.na(role)) %>% 
  group_by(role) %>% 
  summarise(NN=n()) %>% 
  arrange(desc(NN))
head(AllLang, 25)

## # A tibble: 25 x 2
##    role                      NN
##    <chr>                  <int>
##  1 JavaScript            122961
##  2 SQL                    98539
##  3 HTML/CSS               81569
##  4 Python                 71137
##  5 Java                   65662
##  6 C#                     57074
##  7 TypeScript             47955
##  8 PHP                    44112
##  9 C++                    34486
## 10 Bash/Shell/PowerShell  34250
## 11 Bash/Shell             30952
## 12 C                      28988
## 13 HTML                   27672
## 14 CSS                    26477
## 15 Go                     16483
## 16 Node.js                16443
## 17 Ruby                   16095
## 18 Kotlin                 12111
## 19 Swift                  11379
## 20 R                       9303
## 21 VBA                     9026
## 22 Objective-C             8518
## 23 Assembly                7824
## 24 Rust                    7134
## 25 Scala                   7010

ho <- hoTemp %>% 
  select(DatabaseWorkedWith) %>% 
  separate(col=DatabaseWorkedWith,
           paste0("v", str_pad( as.character( seq(1,40,1)), 2, pad="0" )  ),
           sep = ";") 
table(ho$v40)  # To ensure all BLANK

## < table of extent 0 >

AllDB <- ho %>%
  pivot_longer(cols=paste0("v", str_pad( as.character( seq(1,40,1)), 2, pad="0" )  ),
               names_to = "vv",
               values_to = "role") %>% 
  filter(!is.na(role)) %>% 
  group_by(role) %>% 
  summarise(NN=n()) %>% 
  arrange(desc(NN))
head(AllDB, 25)

## # A tibble: 25 x 2
##    role                                            NN
##    <chr>                                        <int>
##  1 MySQL                                        78695
##  2 PostgreSQL                                   60830
##  3 SQLite                                       43837
##  4 MongoDB                                      40784
##  5 Microsoft SQL Server                         40381
##  6 Redis                                        34306
##  7 Elasticsearch                                25564
##  8 MariaDB                                      24937
##  9 Oracle                                       21966
## 10 Firebase                                     15575
## 11 SQL Server                                   14470
## 12 DynamoDB                                      9759
## 13 Cassandra                                     5320
## 14 Other(s):                                     3625
## 15 Microsoft Azure (Tables, CosmosDB, SQL, etc)  2926
## 16 Couchbase                                     2353
## 17 Memcached                                     2130
## 18 Amazon DynamoDB                               1991
## 19 Amazon RDS/Aurora                             1979
## 20 Google Cloud Storage                          1830
## 21 IBM DB2                                       1797
## 22 IBM Db2                                        895
## 23 Neo4j                                          841
## 24 Amazon Redshift                                814
## 25 Google BigQuery                                748

Tidy & Transform

Align values of some variables, e.g. Country, Organization Size (OrgSize),
Select Top 10 countries with the most respondents;
Select full-time employed, independent contractor, freelancer or self-employed respondents who are not full time students;
Respondents should be developers or data experts;
Exclude respondents without providing years of professional coding, education level or income information;
Derive indicators for Developer Type & programming language used;
Re-define the categories of education level (EdLevel), Gender, operating system (OpSys) etc.;

rawdata <- bind_rows(data18a, data19a, data20a, data21a)

rawdata1 <- rawdata %>% 
  mutate(Country=ifelse(Country=="United States of America", "United States", Country)) %>% 
  mutate(Country=ifelse(Country=="United Kingdom of Great Britain and Northern Ireland", "United Kingdom", Country)) %>% 
  select(-starts_with("Assess"), -starts_with("Ads"), -starts_with("JobContact"), 
         -starts_with("JobEmail"), -starts_with("HypotheticalTools")
         ) %>% 
  filter(!is.na(ConvertedCompYearly) & ConvertedCompYearly>0) %>% 
  filter(!is.na(YearsCodingProf) ) %>% 
  filter(!is.na(EdLevel) ) %>% 
  filter(Country %in% Top10_Country) %>% 
  filter(Employment %in% c("Employed full-time", "Independent contractor, freelancer, or self-employed")
         & Student %in% c("No", "Yes, part-time", "Z.Missing")   ) %>% 
  filter(MainBranch %in% c("I am a developer by profession", "Z.Missing")  ) %>% 
  filter(grepl("Data scientist",DevType) |
           grepl("Data or business analyst",DevType) |
           grepl("Engineer, data",DevType) |
           grepl("Engineer, site reliability",DevType) |
           grepl("Scientist",DevType) |
           grepl("Developer",DevType) |
           grepl("developer",DevType)
  ) %>% 
  mutate(year=as_factor(year) ) %>% 
  mutate(OrgSize=str_replace(OrgSize,"2-9","Fewer than 10") ) %>% 
  mutate(OrgSize=str_replace(OrgSize,"2 to 9","Fewer than 10") ) %>% 
  mutate(OrgSize=str_replace(OrgSize,"Just me - I am a freelancer, sole proprietor, etc.", "Fewer than 10 employees") ) %>% 
  mutate(OrgSize=str_replace(OrgSize,"I don't know","Unknown") ) %>% 
  mutate(OrgSize=replace_na(OrgSize,"Unknown") ) %>% 
  mutate(OrgSize=factor(OrgSize, 
                                levels=c("Fewer than 10 employees",
                                         "10 to 19 employees",
                                         "20 to 99 employees",
                                         "100 to 499 employees",
                                         "500 to 999 employees",
                                         "1,000 to 4,999 employees",
                                         "5,000 to 9,999 employees",
                                         "10,000 or more employees",  
                                         "Unknown"))
  ) %>% 
  mutate(fData=ifelse(grepl("Data scientist",DevType) |
                        grepl("Data or business analyst",DevType) |
                        grepl("Engineer, data",DevType) |
                        grepl("Engineer, site reliability",DevType) |
                        grepl("Scientist",DevType), 1, 0)  ) %>% 
  mutate(fDeve=ifelse(grepl("Developer",DevType) |
                        grepl("developer",DevType), 1, 0)  ) %>% 
  filter(fData==1 | fDeve==1 ) %>% 
  mutate(fJavaScript=ifelse(grepl("JavaScript",LanguageWorkedWith), 1, 0)  ) %>% 
  mutate(fSQL=ifelse(grepl("SQL",LanguageWorkedWith), 1, 0)  ) %>% 
  mutate(fPython=ifelse(grepl("Python",LanguageWorkedWith), 1, 0)  ) %>% 
  mutate(fJava=ifelse(grepl("Java;",LanguageWorkedWith), 1, 0)  ) %>% 
  mutate(fRlang=ifelse(grepl("R;",LanguageWorkedWith), 1, 0)  ) %>% 
  mutate(fClang=ifelse(grepl("C#;",LanguageWorkedWith) |
                         grepl("C++;",LanguageWorkedWith, fixed=TRUE) |
                         grepl("C;",LanguageWorkedWith) , 1, 0)  ) 

# Grouping EdLevel
# table(rawdata1$EdLevel)
rawdata2 <- rawdata1 %>% 
  mutate(xEdu=factor(EdLevel, 
                      levels=c("Primary/elementary school",
                               "I never completed any formal education",
                               "Something else",
                               "Secondary school (e.g. American high school, German Realschule or Gymnasium, etc.)",
                               "Associate degree",
                               "Associate degree (A.A., A.S., etc.)",
                               "Some college/university study without earning a degree",
                               "Bachelor’s degree (B.A., B.S., B.Eng., etc.)",
                               "Bachelor’s degree (BA, BS, B.Eng., etc.)",
                               "Master’s degree (M.A., M.S., M.Eng., MBA, etc.)",
                               "Master’s degree (MA, MS, M.Eng., MBA, etc.)",
                               "Other doctoral degree (Ph.D, Ed.D., etc.)",
                               "Other doctoral degree (Ph.D., Ed.D., etc.)",
                               "Professional degree (JD, MD, etc.)"  ),
                      labels=c(rep("1. BelowCollege", 4),
                               rep("2. College",3),
                               rep("3. Degree",2),
                               rep("4. Master",2),
                               rep("5. Doctor",3)  )
                     )
         )
# table(rawdata2$xEdu)

# Grouping Gender & OpSys
# table(rawdata2$Gender); table(rawdata2$OpSys);
rawdata3 <- rawdata2 %>% 
  mutate(xSex=factor(Gender, 
                      levels=c("Man",
                               "Male",
                               "Man;Or, in your own words:",
                               "Woman",
                               "Female",
                               "Woman;Or, in your own words:"
                      ),
                      labels=c(rep("Male", 3),
                               rep("Female",3)
                      )
                     )
         ) %>% 
  mutate(xSex=forcats::fct_explicit_na(xSex, "Others") ) %>% 
  mutate(xOS=factor(OpSys, 
                      levels=c("Windows", "MacOS", "Linux-based" ),
                      labels=c("Windows", "MacOS", "Linux-based" )
                    )
         ) %>% 
  mutate(xOS=forcats::fct_explicit_na(xOS, "Others") )
table(rawdata3$xOS)

## 
##     Windows       MacOS Linux-based      Others 
##       38418       30908       20460        2300

rawdata4 <- rawdata3 %>% 
  mutate(Country = forcats::fct_relevel( as_factor(Country), "United States" ) ) %>% 
  select(year, Country, CurrencySymbol, ConvertedCompYearly,
         OrgSize, YearsCodingProf, xEdu, xSex, xOS,
         fDeve, fData, fJavaScript, fSQL, fPython, fJava, fRlang, fClang,
         DevType, EdLevel, Gender, OpSys, Ethnicity,
         Employment, Student,
         LanguageWorkedWith, DatabaseWorkedWith, Respondent, nCodeExp)

2. DATA EXPLORATION

Remove Exceptionally High Income

Noted that income showed increase in an exponential curve, a number of respondents declared their annual income over USD1M. As data is collected from online voluntary surveys, some respondents might mis-understand the questions and mis-fill the answers. Distributions of coding experience between those with income below & above USD1M are similar, majority has less than 8 years professional coding experience. Even if the exceptionally high income is accurate, it might not be due to their coding experience or education level. Therefore, respondents with income above USD1M will be excluded from this analysis.

rawdata4 %>% 
  ggplot(aes(x=Country, y=ConvertedCompYearly)) +
  geom_boxplot(notch = T) +
  stat_summary(fun.y=mean, geom="point", shape=20, size=5, color="red", fill="red") +
  labs(title="Boxplot Compensation: Too Many Exceptionally High Income",
       subtitle=" - Annual Income as high as USD100M ?")

ho <- rawdata4 %>% 
  arrange(ConvertedCompYearly)
ho$NN <- zoo::index(ho)

ho_interactive <- ho %>% 
  plot_ly(x = ~NN,
          y = ~ConvertedCompYearly,
          type="scatter",
          mode='lines')
htmlwidgets::saveWidget(as_widget(ho_interactive), "index.html")
ho_interactive

ho %>% 
  ggplot(aes(x=NN, y = ConvertedCompYearly ) ) +
  scale_y_continuous(limits= c(0, 25e6)   ) + 
  geom_hline(yintercept = 1e6,  color = "red") +
  annotate("text",  x = 75000,  y = 3e6,
           label = "paste(italic(\"Income \n over $1M\")  )",
           color = "red",  size = 4,  parse = T) +
  annotate("rect",
           fill = "red",   alpha = 0.1,
           xmin = 85000,  xmax = 100000,
           ymin = 1e6,  ymax = 25e6) +
  geom_line(size = 2)

Chart_HiInc <- ho %>% 
  filter(ConvertedCompYearly>=1e6) %>% 
  ggplot(aes(x=YearsCodingProf ) ) +
  geom_bar(aes(y=(..count..)/sum(..count..) ) ) +
  scale_y_continuous(limits=c(0,0.3)) +
  ylab("proportions") +
  labs(title="Income>=$1M" )+
  theme(legend.position = "none",  
      axis.text.x = element_text(angle = 90), 
      text = element_text(family = "Courier")  )
Chart_LoInc <- ho %>% 
  filter(ConvertedCompYearly<1e6) %>% 
  ggplot(aes(x=YearsCodingProf ) ) +
  geom_bar(aes(y=(..count..)/sum(..count..) ) ) +
  scale_y_continuous(limits=c(0,0.3)) +
  ylab("proportions") +
  labs(title="Income < $1M")+
  theme(legend.position = "none",  
        axis.text.x = element_text(angle = 90), 
        text = element_text(family = "Courier")  )
gridExtra::grid.arrange(Chart_LoInc, Chart_HiInc, ncol=2,
                        top = paste0("Extremely High Income Earners has similar", 
                                     "Coding Experience to Others"),
                        bottom = paste0("Even if  high income is accurate, ", 
                                        "it might not be due to their coding experience")
                        )

rawdata5 <- rawdata4 %>% 
  filter(ConvertedCompYearly<1e6)

Detect and Remove Outliers

Even thought respondents with exceptionally high income (>USD1M) are excluded, it is observed that there are still a lot of outliers from boxplot. To remove those outliers, respondents with income within 15 to 85 percentile will be selected for further analysis.

rawdata5 %>% 
  ggplot(aes(x=Country, y=ConvertedCompYearly)) +
  geom_boxplot(notch = T) +
  stat_summary(fun.y=mean, geom="point", shape=20, size=5, color="red", fill="red") +
  labs(title="Boxplot Compensation: A lot of Outliers",
       subtitle=" - To remove reocrds <15 or >85 percentile")

# plot( density(rawdata4$ConvertedCompYearly, na.rm=T) )

remove_outliers <- function(x, na.rm = T, ...) {
  qnt <- quantile(x, probs=c(.15, .85), na.rm = na.rm, ...)
  H <- 0 * IQR(x, na.rm = na.rm)
  y <- x
  y[x < (qnt[1] - H)] <- NA
  y[x > (qnt[2] + H)] <- NA
  y
}
rawdata5$Comp_Clean <- remove_outliers(rawdata5$ConvertedCompYearly)
rawdata6 <- rawdata5 %>%
  filter(!is.na(Comp_Clean) ) %>%
  select(-Comp_Clean)

rawdata6 %>% 
  ggplot(aes(x=Country, y=ConvertedCompYearly)) +
  geom_boxplot(notch = T) +
  stat_summary(fun=mean, geom="point", shape=20, size=5, color="red", fill="red") +
  labs(title="Boxplot Compensation: US & Brazil have the highest income on average",
       subtitle=paste0(" - After excluding below 15 or after 85 percentile",
                       "\n - Still have outliers") )

Inconsistent Survey Data (Exclude 2021 & Poland/Brazil)

When further exploring the data by Slope Graph, it would reveal that average income of Poland/Brazil/India substantially dropped in 2021.

Density Plot would show that they got spike high at low-income in 2021, the distribution is very different from the previous.
Besides, Poland & Brazil have extraordinary fat-tailed for data from 2018-2020, which is very different from other countries.

rawdata6 %>% 
  mutate(year=as.factor(year) ) %>% 
  group_by(year, Country) %>% 
  summarise(mean_comp=round( mean(ConvertedCompYearly) , digits=0) , NN=n() ) %>% 
  ungroup() %>% 
  mutate(year=as.ordered(year)) %>% 
  newggslopegraph(Time=year, Measurement=mean_comp, 
                  Grouping=Country,
                  LineThickness=2, YTextSize=3,
                  LineColor=c("India"="orange", "Brazil"="green", 
                              "Poland"="red", rep("XXX",7) ),
                  Title="Annual Compensation Mean by Country & Year",
                  SubTitle=" - Poland/Brazil/India substantially Dropped in 2021",
                  Caption=NULL
  )

rawdata6 %>% 
  filter(year %in% c(2018, 2019, 2020, 2021) )  %>% 
  mutate(year=as.factor(year)) %>% 
  ggplot(aes(x=ConvertedCompYearly, color=year, fill=year)) + 
  ggplot2::geom_density(alpha=0.2) +
  facet_wrap(.~Country, ncol=5) +
  labs(title="Density Plot of Annual Compensation by Country & Year",
       subtitle=paste0(" - Year 2021 got spike high at low-income for Poland/Brazil/India",
                       "\n - Poland/Brazil fat-tailed at high-income") ) +
    theme(axis.text.x = element_text(angle = 45), 
        text = element_text(family = "Courier")  )

As we could not confirm if these is the true data pattern or data quality issues, we could exclude year 2021 and Poland/Brazil data from our analysis.

rawdata6B <- rawdata6 %>% 
  filter(year!=2021 & ! Country %in% c("Poland", "Brazil")) %>% 
  mutate(Country = droplevels(Country) )

rawdata6B %>% 
  ggplot(aes(x=Country, y=ConvertedCompYearly)) +
  geom_boxplot(notch = T) +
  geom_hline(yintercept=mean(rawdata6B$ConvertedCompYearly),
             color="blue", alpha=0.7) +
  stat_summary(fun=mean, geom="point", shape=20, size=5, color="red", fill="red") +
  labs(title="Boxplot Compensation: US above average",
       subtitle=paste0(" - Other countries look similar") ) +
  theme(legend.position = "none",  
        axis.text.x = element_text(angle = 90), 
        text = element_text(family = "Courier")  )

BoxCox Transformation

bc <- MASS::boxcox(rawdata6B$ConvertedCompYearly ~ rawdata6B$Country)

lambda <- bc$x[which.max(bc$y)]
print(paste0( "By BoxCox Transformation, lambda:", round(lambda, digits=4) ) )

## [1] "By BoxCox Transformation, lambda:-0.0606"

rawdata7 <- rawdata6B %>% 
  mutate(Comp_BC= (ConvertedCompYearly ^lambda-1)/ lambda )

Normality Test

qqnorm(rawdata7$Comp_BC)
qqline(rawdata7$Comp_BC)

Chart7a <- rawdata7 %>% 
  ggplot() + 
  ggplot2::geom_density(aes(x=ConvertedCompYearly), color="red", fill="red", alpha=0.3) +
  xlab("Nominal Income") +
  labs(title="Before Transformation") +
  theme_linedraw()
Chart7b <- rawdata7 %>% 
  ggplot() + 
  ggplot2::geom_density(aes(x=Comp_BC), color="blue", fill="blue", alpha=0.3) +
  xlab("BoxCox Transformed") +
  labs(title="After Transformation") +
  theme_linedraw() 
gridExtra::grid.arrange(Chart7a, Chart7b, ncol = 2, top = "DENSITY PLOT")

rawdata7 %>% lm(ConvertedCompYearly ~ as.factor(Country), .) %>% gvlma::gvlma()

## 
## Call:
## lm(formula = ConvertedCompYearly ~ as.factor(Country), data = .)
## 
## Coefficients:
##                      (Intercept)  as.factor(Country)United Kingdom  
##                           105218                            -32618  
##      as.factor(Country)Australia           as.factor(Country)India  
##                           -22724                            -22294  
##        as.factor(Country)Germany          as.factor(Country)France  
##                           -29432                            -44792  
##         as.factor(Country)Canada     as.factor(Country)Netherlands  
##                           -31036                            -27961  
## 
## 
## ASSESSMENT OF THE LINEAR MODEL ASSUMPTIONS
## USING THE GLOBAL TEST ON 4 DEGREES-OF-FREEDOM:
## Level of Significance =  0.05 
## 
## Call:
##  gvlma::gvlma(x = .) 
## 
##                                   Value p-value                   Decision
## Global Stat        2599.416009052128175 0.00000 Assumptions NOT satisfied!
## Skewness           2591.596422837437331 0.00000 Assumptions NOT satisfied!
## Kurtosis              1.378066632815573 0.24043    Assumptions acceptable.
## Link Function         0.000000000001219 1.00000    Assumptions acceptable.
## Heteroscedasticity    6.441519581873688 0.01115 Assumptions NOT satisfied!

rawdata7 %>% lm(Comp_BC ~ as.factor(Country), .) %>% gvlma::gvlma()

## 
## Call:
## lm(formula = Comp_BC ~ as.factor(Country), data = .)
## 
## Coefficients:
##                      (Intercept)  as.factor(Country)United Kingdom  
##                           8.2849                           -0.1914  
##      as.factor(Country)Australia           as.factor(Country)India  
##                          -0.1211                           -0.1394  
##        as.factor(Country)Germany          as.factor(Country)France  
##                          -0.1605                           -0.2780  
##         as.factor(Country)Canada     as.factor(Country)Netherlands  
##                          -0.1751                           -0.1567  
## 
## 
## ASSESSMENT OF THE LINEAR MODEL ASSUMPTIONS
## USING THE GLOBAL TEST ON 4 DEGREES-OF-FREEDOM:
## Level of Significance =  0.05 
## 
## Call:
##  gvlma::gvlma(x = .) 
## 
##                                   Value p-value                   Decision
## Global Stat        252.2678013143135445  0.0000 Assumptions NOT satisfied!
## Skewness             0.0658450918879243  0.7975    Assumptions acceptable.
## Kurtosis           251.9679907397861598  0.0000 Assumptions NOT satisfied!
## Link Function        0.0000000000002322  1.0000    Assumptions acceptable.
## Heteroscedasticity   0.2339654826392464  0.6286    Assumptions acceptable.

Kurtosis (heavy-tailed) assumption is not satisfied, maybe because data has been truncated by excluding income above USD1M.

3. MODELLING ANALYSIS

Part 1A: Income Differentiation by Country (ANOVA)

Average income among countries are signifcantly different.

AOV_Result <- rawdata7 %>%
  aov(Comp_BC ~ Country, .)
summary(AOV_Result)

##                Df Sum Sq Mean Sq F value              Pr(>F)    
## Country         7    360   51.43    1827 <0.0000000000000002 ***
## Residuals   42706   1202    0.03                                
## ---
## Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1

broom::glance(AOV_Result)

## # A tibble: 1 x 6
##   logLik     AIC     BIC deviance  nobs r.squared
##    <dbl>   <dbl>   <dbl>    <dbl> <int>     <dbl>
## 1 15645. -31272. -31194.    1202. 42714     0.230

Post Hoc Analysis (Tukey HSD)

By pairwise comparison, average income of most countries are different from others.

pairwise_comparison <- TukeyHSD(AOV_Result, which="Country", 
                                order=F, conf.level=.999) %>%
  broom::tidy() %>%
  arrange(desc(estimate) )

pairwise_comparison2 <- pairwise_comparison %>% 
  mutate(pairwise = as.factor(ifelse(adj.p.value>0.001, "C", 
                                     ifelse(estimate<0, "A", "B") ) )   
         ) %>% 
  mutate(estimate2=abs(estimate),
         conf.high2=ifelse(estimate<0,-1*conf.high,conf.high),
         conf.low2=ifelse(estimate<0,-1*conf.low,conf.low)
  )

pairwise_comparison2 %>% 
  ggplot(aes(x = reorder(contrast, estimate),  
             y = estimate2,
             color = pairwise)      ) + 
  geom_point(size = 2) + 
  geom_errorbar(aes(ymin = conf.low2,   ymax = conf.high2),
                width = 0.5) + 
  scale_color_manual(name = "Difference",
                     labels = c( "A" = "Negative",
                                 "B" = "Positive",
                                 "C" = "Insignificant (99.9% CI)"),
                     values = c( "A" = "tomato2",
                                 "B" = "blue",
                                 "C" = "darkgray")
  ) +
  geom_hline(yintercept = 0,  color = "red") +
  xlab("Country Comparison") + 
  theme_bw() + 
  theme(axis.text.x = element_text(angle = 45),
        legend.position = "top") +
  coord_flip() +
  labs(title="Post Hoc Analysis - Tukey HSD",
       subtitle=paste0(" - some country means are close",
                       "\n - e.g. Netherlands, Germany, India, Australia, Canada & UK") )

PART 1B: Net Income / Purchasing Power Differentiation by Country

To better understand the difference of developers’ earning among countries, we would further compare their net income & purchasing power. Net Income is calculated by factoring in the Tax Rate of the corresponding country. Then it is adjusted by Big Mac Index established by The Economist to reflect the Purchasing Power.

Web Scraping

Personal Income Tax Rate is taken from Trading Economics. And the latest updated tax rates (2021) would be applied to 2018-2021 data.
Big Mac Index is taken from the GitHub of The Economist.

library(rvest)

# taxrate <- 
#   "https://tradingeconomics.com/country-list/personal-income-tax-rate" %>% 
#   read_html() %>% 
#   html_node(".table") %>% 
#   html_table() 
# saveRDS(taxrate, file = "LEE_taxrate.rds")
taxrate <- readRDS("LEE_taxrate.rds")
taxrate <- taxrate %>% 
  select(Country, Last, Previous)

# bigmac <- 
#   "https://github.com/TheEconomist/big-mac-data/blob/6c79b1408de38941e7e17c47f7824179bc9e2e2e/output-data/big-mac-full-index.csv" %>% 
#   read_html() %>%  
#   html_node(".js-csv-data") %>% 
#   html_table() 
# saveRDS(bigmac, file = "LEE_bigmac.rds")
bigmac <- readRDS("LEE_bigmac.rds")
bigmac <- bigmac %>% 
  select(-X1)
names(bigmac) <- bigmac[1,]
bigmac <- bigmac[-1,]
bigmac <- bigmac %>% 
  mutate(year = as.numeric(substr(date, 1, 4) ),
         USD_raw=as.numeric(USD_raw) ) %>% 
  filter(year %in% c(2018, 2019, 2020, 2021) & substr(date,6,7)=="07") %>% 
  select(year, name, USD_raw)

Adjusting Income with Tax Rate & Big Mac Index

rawdata7A <- rawdata7 %>%
  mutate(year=as.numeric(as.character(year)),
         name=ifelse(Country=="United Kingdom", "Britain", 
                     ifelse(Country %in% c("France", "Germany", "Netherlands"), "Euro area",
                            as.character(Country) ) )
         ) %>% 
  left_join(taxrate, by = c("Country") ) %>% 
  left_join(bigmac, by = c("year", "name") ) %>% 
  mutate(AfterTaxInc=ConvertedCompYearly * (1 - Last/100),
         BigMacInc = AfterTaxInc / (1+USD_raw),
         Country=forcats::fct_relevel( as_factor(Country), "United States") )

After adjusting with Tax Rate & Big Mac Index, India became the top country with the highest average income (in terms of purchasing power).

Boxplot_OrigInc <- 
  rawdata7A %>% 
  ggplot(aes(x=Country, y=ConvertedCompYearly)) +
  geom_boxplot(notch = T) +
  geom_hline(yintercept=mean(rawdata6B$ConvertedCompYearly),
             color="blue", alpha=0.7) +
  scale_y_continuous(limits = c(20000, 200000)  ) + 
  stat_summary(fun=mean, geom="point", shape=20, size=5, color="black", fill="black") +
  scale_colour_manual(values = c("red",rep("black",7)) ) +
  labs(title="Nominal Income" ) +
  theme(legend.position = "none",
        plot.title = element_text(size=10),
        axis.title.y=element_blank(), axis.title.x=element_blank(),
        axis.text.x = element_text(angle = 90), 
        text = element_text(family = "Courier")  )

Boxplot_AfterTax <-
  rawdata7A %>% 
  ggplot(aes(x=Country, y=AfterTaxInc)) +
  geom_boxplot(notch = T) +
  geom_hline(yintercept=mean(rawdata7A$AfterTaxInc),
             color="blue", alpha=0.7) +
  scale_y_continuous(limits = c(20000, 200000)  ) + 
  stat_summary(fun=mean, geom="point", shape=20, size=5, color="black", fill="black") +
  scale_colour_manual(values = c("red",rep("black",7)) ) +
  labs(title="After Tax Income") +
  theme(legend.position = "none",
        plot.title = element_text(size=10),
        axis.title.y=element_blank(), axis.title.x=element_blank(),
        axis.text.x = element_text(angle = 90), 
        text = element_text(family = "Courier")  )

Boxplot_BigMac <-
  rawdata7A %>% 
  ggplot(aes(x=Country, y=BigMacInc, color=Country)) +
  geom_boxplot(notch = T) +
  geom_hline(yintercept=mean(rawdata7A$BigMacInc),
             color="blue", alpha=0.7) +
  scale_y_continuous(limits = c(20000, 200000)  ) + 
  stat_summary(fun=mean, geom="point", shape=20, size=5) +
  scale_colour_manual(values = c(rep("black",3),"red",rep("black",4)) ) +
  labs(title="After Tax & Big Mac Index" ) +
  theme(legend.position = "none",
        plot.title = element_text(size=10),
        axis.title.y=element_blank(), axis.title.x=element_blank(),
        axis.text.x = element_text(angle = 90), 
        text = element_text(family = "Courier")  )

gridExtra::grid.arrange(Boxplot_OrigInc, Boxplot_AfterTax, Boxplot_BigMac, 
                        ncol=3, left="Annual Income",
                        bottom="Country",
                        top="Income Looks Different After Tax & Adjusted with Big Mac Index")

PART 2: Interplay Education & Coding Experience on Income (for US Only)

As shown in Part 1 that there is significant difference of income among countries, hence we will only focus on the country of United States which has the most respondents in this survey.

As YearsCodingProf had been grouped as a categorical variable in year 2018, we will only use 2019-2021 survey data to conduct analysis in Part 2.

table(data18$YearsCodingProf )

## 
##        0-2 years      12-14 years      15-17 years      18-20 years 
##            23421             4287             3012             2830 
##      21-23 years      24-26 years      27-29 years        3-5 years 
##             1368              857              506            21362 
## 30 or more years        6-8 years       9-11 years 
##             1302            11385             7573

USbase <- rawdata6 %>% 
  filter(year!=2018 & Country=="United States"
         & nCodeExp!=99) %>% 
  mutate(oCodeExp=nCodeExp) %>% 
  mutate_at(vars( c(nCodeExp) ),
            ~(. - mean(., na.rm = T) )
            )
# mean(USbase$oCodeExp)

BoxCox Transformation for US

bc <- MASS::boxcox(USbase$ConvertedCompYearly ~ 1)

lambda_US <- bc$x[which.max(bc$y)]
print(paste0( "By BoxCox Transformation, lambda:", round(lambda_US, digits=4) ) )

## [1] "By BoxCox Transformation, lambda:0.5859"

USbase <- USbase %>% 
  mutate(Comp_US= (ConvertedCompYearly ^lambda_US-1)/ lambda_US )

US Model 1: Interaction with Python Language

It is observed from the below linear modeling output that:

Impact of Below College & College is indifferent, given that the 95% C.I. of coefficient estimate of Below College contains 0;
Impact of Master & Doctor to income may be similar given that their coefficient estimates are very similar;
On average Female may earn less;
Developer using Python may earn more;
No change in the impacts of Education Level / Gender after adjusting for Python;

USlm1 <-  USbase %>% 
  lm(Comp_US ~ (nCodeExp + xEdu + xSex) * fPython , .)
summary(USlm1)

## 
## Call:
## lm(formula = Comp_US ~ (nCodeExp + xEdu + xSex) * fPython, data = .)
## 
## Residuals:
##      Min       1Q   Median       3Q      Max 
## -1123.58  -187.99    -9.93   189.85   723.12 
## 
## Coefficients:
##                         Estimate Std. Error t value             Pr(>|t|)    
## (Intercept)            1443.9735    14.0139 103.039 < 0.0000000000000002 ***
## nCodeExp                 12.4601     0.2670  46.673 < 0.0000000000000002 ***
## xEdu2. College          -11.4552    15.0161  -0.763             0.445555    
## xEdu3. Degree            50.1827    14.3045   3.508             0.000452 ***
## xEdu4. Master           110.9655    15.2820   7.261    0.000000000000397 ***
## xEdu5. Doctor            99.1840    24.1411   4.109    0.000039968683609 ***
## xSexFemale              -35.6747     7.9216  -4.503    0.000006720755139 ***
## xSexOthers               -3.4947    10.6568  -0.328             0.742963    
## fPython                  65.2063    23.0058   2.834             0.004596 ** 
## nCodeExp:fPython          0.5181     0.4263   1.215             0.224305    
## xEdu2. College:fPython  -13.1711    24.7197  -0.533             0.594165    
## xEdu3. Degree:fPython   -29.5913    23.4319  -1.263             0.206651    
## xEdu4. Master:fPython   -33.6110    24.5523  -1.369             0.171028    
## xEdu5. Doctor:fPython   -12.6106    33.3084  -0.379             0.704987    
## xSexFemale:fPython        3.0483    12.7931   0.238             0.811667    
## xSexOthers:fPython       11.2847    16.4352   0.687             0.492331    
## ---
## Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
## 
## Residual standard error: 258.4 on 21641 degrees of freedom
## Multiple R-squared:  0.1676, Adjusted R-squared:  0.167 
## F-statistic: 290.4 on 15 and 21641 DF,  p-value: < 0.00000000000000022

ho <- USlm1 %>% broom::tidy()
broom::glance(USlm1)

## # A tibble: 1 x 12
##   r.squared adj.r.squared sigma statistic p.value    df   logLik     AIC     BIC
##       <dbl>         <dbl> <dbl>     <dbl>   <dbl> <dbl>    <dbl>   <dbl>   <dbl>
## 1     0.168         0.167  258.      290.       0    15 -151016. 302066. 302202.
##      deviance df.residual  nobs
##         <dbl>       <int> <int>
## 1 1445013783.       21641 21657

US Model 2: Interaction with Education Level

Based on the observation in US Model 1, we group Below College & College together as Without Degree, and group Master & Doctor together as Postgraduate. And run the 2nd linear model with interaction between Education Level & Coding Experience.

USbase2 <- USbase %>% 
  mutate(xEdu2=factor(xEdu, 
                   levels=c("1. BelowCollege",
                            "2. College",
                            "3. Degree",
                            "4. Master",
                            "5. Doctor"  ),
                   labels=c(rep("1. Without Degree", 2),
                            rep("2. Undergraduate",1),
                            rep("3. Postgraduate",2)  )  ) 
         )

USlm2 <-  USbase2 %>% 
  lm(Comp_US ~ nCodeExp * xEdu2 + xSex + fPython , .)
summary(USlm2)

## 
## Call:
## lm(formula = Comp_US ~ nCodeExp * xEdu2 + xSex + fPython, data = .)
## 
## Residuals:
##      Min       1Q   Median       3Q      Max 
## -1142.74  -187.84    -9.44   190.72   724.43 
## 
## Coefficients:
##                                 Estimate Std. Error t value
## (Intercept)                    1437.7990     4.3687 329.114
## nCodeExp                         13.4079     0.4324  31.007
## xEdu22. Undergraduate            55.4926     4.6914  11.828
## xEdu23. Postgraduate            117.6905     5.8229  20.212
## xSexFemale                      -34.9788     6.2100  -5.633
## xSexOthers                        2.4010     8.0993   0.296
## fPython                          39.0994     3.5892  10.893
## nCodeExp:xEdu22. Undergraduate    0.2286     0.5119   0.447
## nCodeExp:xEdu23. Postgraduate    -4.0361     0.6173  -6.539
##                                            Pr(>|t|)    
## (Intercept)                    < 0.0000000000000002 ***
## nCodeExp                       < 0.0000000000000002 ***
## xEdu22. Undergraduate          < 0.0000000000000002 ***
## xEdu23. Postgraduate           < 0.0000000000000002 ***
## xSexFemale                          0.0000000179645 ***
## xSexOthers                                    0.767    
## fPython                        < 0.0000000000000002 ***
## nCodeExp:xEdu22. Undergraduate                0.655    
## nCodeExp:xEdu23. Postgraduate       0.0000000000634 ***
## ---
## Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
## 
## Residual standard error: 258 on 21648 degrees of freedom
## Multiple R-squared:  0.1699, Adjusted R-squared:  0.1696 
## F-statistic:   554 on 8 and 21648 DF,  p-value: < 0.00000000000000022

ho <- USlm2 %>% broom::tidy()
broom::glance(USlm2)

## # A tibble: 1 x 12
##   r.squared adj.r.squared sigma statistic p.value    df   logLik     AIC     BIC
##       <dbl>         <dbl> <dbl>     <dbl>   <dbl> <dbl>    <dbl>   <dbl>   <dbl>
## 1     0.170         0.170  258.      554.       0     8 -150985. 301990. 302070.
##      deviance df.residual  nobs
##         <dbl>       <int> <int>
## 1 1440888661.       21648 21657

Chart of Estimates

In the 2nd linear model, noted that Undergrade has similar interaction with Coding Experience as Without Degree; and Gender Others may be indifferent from Male.

jtools::plot_summs(USlm1,
           USlm2,
           scale = T,
           robust = list(FALSE,
                         "HC3"),
           model.names = c("Model 1 Interaction with Python",
                           "Model 2 Interaction with Edu Level"),
           plot.distributions = T)

## Loading required namespace: broom.mixed

Adjusted for Education Level, controlling fPython & xSex

predicted_Y_values_tbl <- USbase2 %>% # input our dataset
  modelr::data_grid(nCodeExp, # Explanatory Variable = X1 
                    xEdu2, # Moderator = X2
                    fPython = 0.5,    # take 0.5 for control
                    xSex = "Male" ) %>%  # take mode
  add_predictions(USlm2) %>% 
  rename(predicted_Y = pred)

# predicted_Y_values_tbl %>% print(n = nrow(.) )

To undid the centering of Coding Experience (nCodeExp) and transforming of predicted income (predicted_Y). The interplay charts showing that Postgraduate may start with higher income, but their rate of increment is lower than Undergraduate.

predicted_Y_values_tbl <- predicted_Y_values_tbl %>% 
  mutate(nCodeExp = nCodeExp + mean(USbase2$oCodeExp),
         oPredict_Y = ( predicted_Y*lambda_US + 1 )^(1/lambda_US)
  )

predicted_Y_values_tbl %>% 
  ggplot(aes(x = nCodeExp, y = oPredict_Y))+
  geom_point(data=USbase2, aes(x=oCodeExp, y=ConvertedCompYearly, color=factor(xEdu2)) , alpha=0.1)+
  geom_line(aes(y=oPredict_Y,color=factor(xEdu2)) ,size=2)+
  labs(x = "Years of Professional Coding", y = "Annual Income",
       color= "Edu Level") +
  facet_wrap(~factor(xEdu2) ) +
  theme(legend.position="none") +
  labs(title="The Interplay of Education and Coding Experience on Annual Income",
       subtitle=paste0("Postgrad got higher income at the early stage, ",
                       "but lower rate of increment than Undergrad") )

US Model 3: Interaction with Education Level (Undergrad vs Postgrad)

Next step we will focus on comparing Postgraduate and Undergraduate, to see their income changing with years of professional coding experience.

As expected, Gender Others is indifferent from Male and Postgraduate (fPostgrad) usually has a positive impact to income. However, interaction term has negative estimate, which means Postgraduate’s income would increase with coding experience slower when compared to Undergradate.

USbase3 <- USbase %>% 
  filter(!xEdu %in% c("1. BelowCollege", "2. College") ) %>% 
  mutate(fPostgrad=ifelse(xEdu=="3. Degree", 0, 1 ) )

USlm3 <-  USbase3 %>% 
  lm(Comp_US ~ nCodeExp * fPostgrad + xSex + fPython , .)
summary(USlm3)

## 
## Call:
## lm(formula = Comp_US ~ nCodeExp * fPostgrad + xSex + fPython, 
##     data = .)
## 
## Residuals:
##     Min      1Q  Median      3Q     Max 
## -1135.7  -186.3   -10.9   187.3   685.8 
## 
## Coefficients:
##                     Estimate Std. Error t value             Pr(>|t|)    
## (Intercept)        1494.6485     2.8659 521.525 < 0.0000000000000002 ***
## nCodeExp             13.6194     0.2734  49.818 < 0.0000000000000002 ***
## fPostgrad            62.5186     4.5961  13.602 < 0.0000000000000002 ***
## xSexFemale          -30.6403     6.5558  -4.674           0.00000298 ***
## xSexOthers           -1.7697     8.8863  -0.199                0.842    
## fPython              35.3884     3.9056   9.061 < 0.0000000000000002 ***
## nCodeExp:fPostgrad   -4.2553     0.5122  -8.309 < 0.0000000000000002 ***
## ---
## Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
## 
## Residual standard error: 254.3 on 17593 degrees of freedom
## Multiple R-squared:  0.1633, Adjusted R-squared:  0.163 
## F-statistic: 572.2 on 6 and 17593 DF,  p-value: < 0.00000000000000022

ho <- USlm3 %>% broom::tidy()
broom::glance(USlm3)

## # A tibble: 1 x 12
##   r.squared adj.r.squared sigma statistic p.value    df   logLik     AIC     BIC
##       <dbl>         <dbl> <dbl>     <dbl>   <dbl> <dbl>    <dbl>   <dbl>   <dbl>
## 1     0.163         0.163  254.      572.       0     6 -122450. 244915. 244978.
##      deviance df.residual  nobs
##         <dbl>       <int> <int>
## 1 1137977561.       17593 17600

Adjusted for Education Level, controlling fPython & xSex

predicted_Y_values_tbl <- USbase3 %>% # input our dataset
  modelr::data_grid(nCodeExp, # Explanatory Variable = X1 
                    fPostgrad, # Moderator = X2
                    fPython = 0.5,    # take 0.5 for control
                    xSex = "Male" ) %>%  # take mode
  add_predictions(USlm3) %>% 
  rename(predicted_Y = pred)

# predicted_Y_values_tbl %>% print(n = nrow(.)  )

# We undid the centering of variable (Coding Experience).
predicted_Y_values_tbl <- predicted_Y_values_tbl %>% 
  mutate(nCodeExp = nCodeExp + mean(USbase3$oCodeExp),
         oPredict_Y = ( predicted_Y*lambda_US + 1 )^(1/lambda_US)
  )

Step 1: Plot interaction using interact_plot()

interactions::interact_plot(USlm3, # plug in your model
                            pred = nCodeExp, # X1 = Explanatory variable
                            modx = fPostgrad, # X2 = Moderator
                            modx.labels = c("Undergrad", # = 0
                                            "Postgrad"), # = 1
                            legend.main = "Edu Level",
                            interval = T, # display Confidence Intervals
                            int.width = 0.95,
                            colors = c("tomato3",
                                       "deepskyblue3")
) + 
  theme(legend.position = "top")

Step 2: Run simple slopes analysis

interactions::sim_slopes(USlm3,
                         pred = nCodeExp, # x-axis
                         modx = fPostgrad, # legend
                         johnson_neyman = F) # set J-N intervals at F

## SIMPLE SLOPES ANALYSIS 
## 
## Slope of nCodeExp when fPostgrad = 0.00 (0): 
## 
##    Est.   S.E.   t val.      p
## ------- ------ -------- ------
##   13.62   0.27    49.82   0.00
## 
## Slope of nCodeExp when fPostgrad = 1.00 (1): 
## 
##   Est.   S.E.   t val.      p
## ------ ------ -------- ------
##   9.36   0.44    21.46   0.00

Step 3: Spotlight analysis (set J-N at T; SWITCH pred and modx)

interactions::sim_slopes(USlm3,
                         pred = fPostgrad,
                         modx = nCodeExp,
                         johnson_neyman = T) # set J-N intervals at T

## JOHNSON-NEYMAN INTERVAL 
## 
## When nCodeExp is OUTSIDE the interval [11.40, 19.63], the slope of
## fPostgrad is p < .05.
## 
## Note: The range of observed values of nCodeExp is [-9.69, 40.31]
## 
## SIMPLE SLOPES ANALYSIS 
## 
## Slope of fPostgrad when nCodeExp = -8.7864122 (- 1 SD): 
## 
##    Est.   S.E.   t val.      p
## ------- ------ -------- ------
##   99.91   6.68    14.96   0.00
## 
## Slope of fPostgrad when nCodeExp = -0.3913421 (Mean): 
## 
##    Est.   S.E.   t val.      p
## ------- ------ -------- ------
##   64.18   4.62    13.90   0.00
## 
## Slope of fPostgrad when nCodeExp =  8.0037280 (+ 1 SD): 
## 
##    Est.   S.E.   t val.      p
## ------- ------ -------- ------
##   28.46   5.91     4.81   0.00

Step 4: Run interaction_plot() again by adding regions of significance

# ```{r fig.align = "center", fig.width = 10, fig.height= 10, warning = F}
interactions::interact_plot(USlm3, # plug in your model
              pred = nCodeExp, # X1 = Explanatory variable
              modx = fPostgrad, # X2 = Moderator
              legend.main = "Education Level",
              modx.labels = c("Undergrad", # = 0
                              "Postgrad"), # = 1
              interval = T, # display Confidence Intervals
              int.width = 0.95,
              colors = c("tomato3",
                         "deepskyblue3"),
              plot.points = T, # REVEAL your data points
              point.alpha = 0.1,
              jitter = 0.2
) + 
  # Set benchmark for regrions of significance  11.40, 19.63
  geom_vline(xintercept = 11.4, color = "red", size = 2) + 
  geom_vline(xintercept = 19.63, color = "red", size = 2) + 
  scale_y_continuous(limits = c(1200, 2100)  ) + 
  annotate("rect",
           fill = "yellow",
           alpha = 0.1,
           xmin = -10,
           xmax = 11.4,
           ymin = 1200,
           ymax = 2100) +
  annotate("rect",
           fill = "yellow",
           alpha = 0.1,
           xmin = 19.63,
           xmax = 40,
           ymin = 1200,
           ymax = 2100) +
  annotate("text",
           x = 30,
           y = 1400,
           label = "Regions of significance\nby Johnson-Neyman Analysis \nwith alpha at 5%") +
  labs(title="Interplay of Education and Coding Experience on Annual Income",
       subtitle=paste0("Postgraduate got higher income at the early stage, ",
                       "\nbut became lower than Undergraduate after 28.9 years of coding experience" ),
       x = paste0("Years of Professional Coding (Centered, mean=", 
                  round(mean(USbase3$oCodeExp), digits=2),")" ),
       y = "Annual Compensation (BoxCox Transformed)") + 
  theme(legend.position = "top",
        text = element_text(family = "Courier"),
        axis.title = element_text()
  )

Specify OLS model

We ran three regression models for US respondents. The 1st model regressed coding experience, education level, gender and usage of Python, along with interaction terms with usage of Python, onto BoxCox Transformed Income (USlm1).

\[ \operatorname{Comp\_US} = \alpha + \beta_{1}(\operatorname{nCodeExp}) + \beta_{2}(\operatorname{xEdu}_{\operatorname{2.\ College}}) + \beta_{3}(\operatorname{xEdu}_{\operatorname{3.\ Degree}}) + \beta_{4}(\operatorname{xEdu}_{\operatorname{4.\ Master}}) + \beta_{5}(\operatorname{xEdu}_{\operatorname{5.\ Doctor}}) + \beta_{6}(\operatorname{xSex}_{\operatorname{Female}}) \\+ \beta_{7}(\operatorname{xSex}_{\operatorname{Others}}) + \beta_{8}(\operatorname{fPython}) + \beta_{9}(\operatorname{nCodeExp} \times \operatorname{fPython}) + \beta_{10}(\operatorname{xEdu}_{\operatorname{2.\ College}} \times \operatorname{fPython}) + \beta_{11}(\operatorname{xEdu}_{\operatorname{3.\ Degree}} \times \operatorname{fPython}) \\+ \beta_{12}(\operatorname{xEdu}_{\operatorname{4.\ Master}} \times \operatorname{fPython}) + \beta_{13}(\operatorname{xEdu}_{\operatorname{5.\ Doctor}} \times \operatorname{fPython}) + \beta_{14}(\operatorname{xSex}_{\operatorname{Female}} \times \operatorname{fPython}) + \beta_{15}(\operatorname{xSex}_{\operatorname{Others}} \times \operatorname{fPython}) + \epsilon \]

As the 1st model showed no interaction of usage of Python with other independent variables, We ran the 2nd model which regressed coding experience, education level, gender and usage of Python, along with interaction term between coding experience and education level, onto BoxCox Transformed Income (USlm2).

\[ \operatorname{Comp\_US} = \alpha + \beta_{1}(\operatorname{nCodeExp}) + \beta_{2}(\operatorname{xEdu2}_{\operatorname{2.\ Undergraduate}}) + \beta_{3}(\operatorname{xEdu2}_{\operatorname{3.\ Postgraduate}}) + \beta_{4}(\operatorname{xSex}_{\operatorname{Female}}) + \beta_{5}(\operatorname{xSex}_{\operatorname{Others}}) \\+ \beta_{6}(\operatorname{fPython}) + \beta_{7}(\operatorname{nCodeExp} \times \operatorname{xEdu2}_{\operatorname{2.\ Undergraduate}}) + \beta_{8}(\operatorname{nCodeExp} \times \operatorname{xEdu2}_{\operatorname{3.\ Postgraduate}}) + \epsilon \]

The last model is similar to the 2nd, but we focus on comparing Postgraduate and Undergraduate, to see their income changing with years of professional coding experience. The 3rd model regressed coding experience, gender and usage of Python, along with interaction term between coding experience and postgrad indicator, onto BoxCox Transformed Income (USlm3).

\[ \operatorname{Comp\_US} = \alpha + \beta_{1}(\operatorname{nCodeExp}) + \beta_{2}(\operatorname{fPostgrad}) + \beta_{3}(\operatorname{xSex}_{\operatorname{Female}}) + \beta_{4}(\operatorname{xSex}_{\operatorname{Others}}) \\+ \beta_{5}(\operatorname{fPython}) + \beta_{6}(\operatorname{nCodeExp} \times \operatorname{fPostgrad}) + \epsilon \]

Model Results

Model 1 for US

library(knitr) 
library(kableExtra)

## 
## Attaching package: 'kableExtra'

## The following object is masked from 'package:dplyr':
## 
##     group_rows

broom::tidy(USlm1)%>%
  kbl(caption = "US Model 1") %>% 
  kable_styling("striped", full_width = T, fixed_thead = T, font_size = 12) %>%
  column_spec(c(1, 5), bold = T) %>%
  row_spec(c(3, 8, 10,11,12,13,14,15,16), bold = F, color = "darkgray")

US Model 1
term	estimate	std.error	statistic	p.value
(Intercept)	1443.9734970	14.0139153	103.0385491	0.0000000
nCodeExp	12.4600696	0.2669671	46.6726850	0.0000000
xEdu2. College	-11.4552131	15.0161488	-0.7628596	0.4455555
xEdu3. Degree	50.1826753	14.3044855	3.5081776	0.0004521
xEdu4. Master	110.9655227	15.2819523	7.2612138	0.0000000
xEdu5. Doctor	99.1840375	24.1411277	4.1085089	0.0000400
xSexFemale	-35.6746869	7.9216310	-4.5034523	0.0000067
xSexOthers	-3.4947368	10.6567722	-0.3279358	0.7429634
fPython	65.2063304	23.0058058	2.8343424	0.0045963
nCodeExp:fPython	0.5180892	0.4263423	1.2151955	0.2243048
xEdu2. College:fPython	-13.1710921	24.7197064	-0.5328175	0.5941654
xEdu3. Degree:fPython	-29.5913374	23.4318938	-1.2628658	0.2066510
xEdu4. Master:fPython	-33.6110053	24.5523240	-1.3689541	0.1710278
xEdu5. Doctor:fPython	-12.6106231	33.3083649	-0.3786023	0.7049869
xSexFemale:fPython	3.0483107	12.7930664	0.2382783	0.8116675
xSexOthers:fPython	11.2847142	16.4352276	0.6866175	0.4923312

kable(broom::glance(USlm1))%>%
  kable_styling("striped", full_width = T, font_size = 12) %>%
  column_spec(c(2, 4, 6, 8, 10, 12), bold = T, color = "white", background = "#ff6347")

r.squared	adj.r.squared	sigma	statistic	p.value	df	logLik	AIC	BIC	deviance	df.residual	nobs
0.1675688	0.1669918	258.4029	290.4228	0	15	-151016.2	302066.4	302202.1	1445013783	21641	21657

Model 2 for US

broom::tidy(USlm2)%>%
  kbl(caption = "US Model 2") %>% 
  kable_styling("striped", full_width = T, fixed_thead = T, font_size = 12) %>%
  column_spec(c(1, 5), bold = T) %>%
  row_spec(c(6, 8), bold = F, color = "darkgray")

US Model 2
term	estimate	std.error	statistic	p.value
(Intercept)	1437.799039	4.3686993	329.1137579	0.0000000
nCodeExp	13.407912	0.4324132	31.0071754	0.0000000
xEdu22. Undergraduate	55.492579	4.6914335	11.8284910	0.0000000
xEdu23. Postgraduate	117.690532	5.8228831	20.2117285	0.0000000
xSexFemale	-34.978754	6.2099793	-5.6326683	0.0000000
xSexOthers	2.400981	8.0992819	0.2964437	0.7668941
fPython	39.099434	3.5892462	10.8934945	0.0000000
nCodeExp:xEdu22. Undergraduate	0.228564	0.5118888	0.4465111	0.6552325
nCodeExp:xEdu23. Postgraduate	-4.036089	0.6172565	-6.5387547	0.0000000

kable(broom::glance(USlm2))%>%
  kable_styling("striped", full_width = T, font_size = 12) %>%
  column_spec(c(2, 4, 6, 8, 10, 12), bold = T, color = "white", background = "#ff6347")

r.squared	adj.r.squared	sigma	statistic	p.value	df	logLik	AIC	BIC	deviance	df.residual	nobs
0.1699451	0.1696384	257.992	554.0254	0	8	-150985.2	301990.5	302070.3	1440888661	21648	21657

Model 3 for US

broom::tidy(USlm3)%>%
  kbl(caption = "US Model 3") %>% 
  kable_styling("striped", full_width = T, fixed_thead = T, font_size = 12) %>%
  column_spec(c(1, 5), bold = T) %>%
  row_spec(c(5), bold = F, color = "darkgray")

US Model 3
term	estimate	std.error	statistic	p.value
(Intercept)	1494.648523	2.8659174	521.5253347	0.000000
nCodeExp	13.619403	0.2733816	49.8182970	0.000000
fPostgrad	62.518650	4.5961217	13.6024792	0.000000
xSexFemale	-30.640344	6.5558317	-4.6737539	0.000003
xSexOthers	-1.769707	8.8862643	-0.1991509	0.842147
fPython	35.388359	3.9056070	9.0609115	0.000000
nCodeExp:fPostgrad	-4.255334	0.5121597	-8.3086082	0.000000

kable(broom::glance(USlm3))%>%
  kable_styling("striped", full_width = T, font_size = 12) %>%
  column_spec(c(2, 4, 6, 8, 10, 12), bold = T, color = "white", background = "#ff6347")

r.squared	adj.r.squared	sigma	statistic	p.value	df	logLik	AIC	BIC	deviance	df.residual	nobs
0.163277	0.1629916	254.3296	572.1788	0	6	-122449.7	244915.4	244977.7	1137977561	17593	17600

Interpretation of the Results

From our data science project, we could find the following two findings:

Nominal Income of developers among countries are significantly different; based on the survey data and out of the countries under our study. US has the highest while France has the lowest income for developers. After considering the personal income tax rates, the ranking is still similar that US has the highest while France has the lowest after tax income for developers; noted that Germany & Netherlands who used to have slightly higher nominal income than Canada, but their after tax income would become lower than Canada. If factoring in Big Mac Index to reflect their purchasing power, India would be the highest and far above the next highest country (US).
Regarding the interplay of , if only sampling US respondents for further analysis, we could observe:

Impact of Below College & College is indifferent, and impact of Master & Doctor to income may be similar
It is sad to say this but shown by US Model 1 (USlm1) that on average Female may earn less, and developers using Python may earn more (while this project is conducted by R).
As expected, US Model 3 (USlm3) has shown that coding experience will have positive impact to income and postgraduate will be better than Undgergraduate. However, interestingly the interaction term between postgraduate and coding experience has negative impact to income, which means income increase with coding experience for postgraduate would be slower than undergraduate, even though postgraduate starts at a higher income level, undergraduate might catch up after about 28.9 years of professional coding experience.

Implications

In terms of purchasing power, developers working in India earn much higher than other countries, but the 2nd best country (US) may be a better option for developers who want to migrate to other countries, as US on average has the highest after tax income, considering that income is not just for paying living expenses, it could also be used to exploit investment opportunites around the world.
US Model 3 (USlm3) showed that in the long run (after 28.9 years) postgraduate developers may earn less than undergraduate; but this conclusion came from observing different subjects with different years of coding experience at specific snapshots 2019-2021 (cross-sectional study), instead of by repeatedly measuring the same pool of subjects over time. Their academic major would be playing a more critical role; what postgraduate learnt in 30 years ago might not be very relevant to the challenges faced by developers or data scientists today, hence we could see postgradate with less experience earns more, given they just start their career, what they learnt should be the most updated and fitted to the job market demands. Moreover it must be cautious when investigating their causalty relationship; it could be because developers with slower career progress tried to take further studies to enhance their market values while it might be ineffective. Besides, it could be because of other confounders not considered in the model, for instance, postgraduate took some more times to obtain their Master/Doctor degree and started their career later; even though both of them have the same 30 years of coding experience, postgraduate would be older and might have less opportunities than undergraduate developers. It is interesting that data reveals something different from layman expectation, but further study would be required to understand why this happened.

Limitations and Future Directions

There is no comprehensive adjustment indicator to truly reflect the purchasing power. While ingredients & standards of Big Mac may be similar across countries, consumption of Big Mac may only occupy a small portion of personal income. And market demands may distort the price of Big Mac too; for instance, as a number of indian are vegetarian or not having beef, Big Mac may not be a common good targeting the general public in india. Therefore, different from other countries, Big Mac Index may not be a proper measure of Indian purchasing power. For developed countries & metropolis, a significant portion of personal income in general may be spent on housing/rents, housing price & rents may be considered to adjust the income, while property price sometimes could be over-valued due to speculation.
By right India would have more lower-income deverlopers, but a lot of them would have been removed as outliers when we identify outliers based on the respondents of the Top 10 countries of which many are from Western countries. In the future we may consider to remove outliers by country by country.
For simplicity, Tax Rates updated in 2021 are applied to the survey data collected in 2018 - 2020.
Conclusion of the review is only based on the survey results, which might not be used to inference the population given that number of respondents was relatively small and a number of them did not provide income information in survey.

sessionInfo()

sessionInfo()

## R version 4.1.0 (2021-05-18)
## Platform: x86_64-w64-mingw32/x64 (64-bit)
## Running under: Windows 10 x64 (build 22000)
## 
## Matrix products: default
## 
## locale:
## [1] LC_COLLATE=English_Hong Kong SAR.1252 
## [2] LC_CTYPE=English_Hong Kong SAR.1252   
## [3] LC_MONETARY=English_Hong Kong SAR.1252
## [4] LC_NUMERIC=C                          
## [5] LC_TIME=English_Hong Kong SAR.1252    
## 
## attached base packages:
## [1] stats     graphics  grDevices utils     datasets  methods   base     
## 
## other attached packages:
##  [1] kableExtra_1.3.4        knitr_1.33              rvest_1.0.1            
##  [4] equatiomatic_0.3.0.9000 CGPfunctions_0.6.3      plotly_4.10.0          
##  [7] broom_0.7.9             modelr_0.1.8            lubridate_1.7.10       
## [10] forcats_0.5.1           stringr_1.4.0           dplyr_1.0.7            
## [13] purrr_0.3.4             readr_2.0.1             tidyr_1.1.3            
## [16] tibble_3.1.4            ggplot2_3.3.5           tidyverse_1.3.1        
## 
## loaded via a namespace (and not attached):
##   [1] readxl_1.3.1           backports_1.2.1        jtools_2.1.4          
##   [4] systemfonts_1.0.3      lazyeval_0.2.2         splines_4.1.0         
##   [7] crosstalk_1.1.1        digest_0.6.27          interactions_1.1.5    
##  [10] htmltools_0.5.2        fansi_0.5.0            magrittr_2.0.1        
##  [13] paletteer_1.4.0        tzdb_0.1.2             svglite_2.0.0         
##  [16] sandwich_3.0-1         colorspace_2.0-2       ggrepel_0.9.1         
##  [19] haven_2.4.3            xfun_0.25              crayon_1.4.1          
##  [22] jsonlite_1.7.2         libcoin_1.0-9          Exact_3.0             
##  [25] lme4_1.1-27.1          survival_3.2-11        zoo_1.8-9             
##  [28] glue_1.4.2             gtable_0.3.0           emmeans_1.7.0         
##  [31] webshot_0.5.2          MatrixModels_0.5-0     sjstats_0.18.1        
##  [34] sjmisc_2.8.7           scales_1.1.1           mvtnorm_1.1-2         
##  [37] DBI_1.1.1              Rcpp_1.0.7             viridisLite_0.4.0     
##  [40] xtable_1.8-4           performance_0.8.0      ggstance_0.3.5        
##  [43] proxy_0.4-26           Formula_1.2-4          datawizard_0.2.1      
##  [46] htmlwidgets_1.5.4      httr_1.4.2             ellipsis_0.3.2        
##  [49] pkgconfig_2.0.3        farver_2.1.0           sass_0.4.0            
##  [52] dbplyr_2.1.1           utf8_1.2.2             tidyselect_1.1.1      
##  [55] labeling_0.4.2         rlang_0.4.11           later_1.3.0           
##  [58] effectsize_0.5         munsell_0.5.0          cellranger_1.1.0      
##  [61] tools_4.1.0            cli_3.0.1              generics_0.1.1        
##  [64] pacman_0.5.1           sjlabelled_1.1.8       evaluate_0.14         
##  [67] fastmap_1.1.0          yaml_2.2.1             rematch2_2.1.2        
##  [70] fs_1.5.0               pander_0.6.4           ggmosaic_0.3.3        
##  [73] rootSolve_1.8.2.3      pbapply_1.5-0          nlme_3.1-152          
##  [76] mime_0.12              xml2_1.3.2             compiler_4.1.0        
##  [79] rstudioapi_0.13        e1071_1.7-9            reprex_2.0.1          
##  [82] bslib_0.3.1            DescTools_0.99.43      stringi_1.7.4         
##  [85] highr_0.9              parameters_0.15.0      lattice_0.20-44       
##  [88] Matrix_1.3-3           nloptr_1.2.2.2         vctrs_0.3.8           
##  [91] pillar_1.6.4           lifecycle_1.0.1        jquerylib_0.1.4       
##  [94] estimability_1.3       data.table_1.14.0      insight_0.14.5        
##  [97] lmom_2.8               httpuv_1.6.2           R6_2.5.1              
## [100] promises_1.2.0.1       gridExtra_2.3          BayesFactor_0.9.12-4.2
## [103] gld_2.6.2              boot_1.3-28            MASS_7.3-54           
## [106] gtools_3.9.2           assertthat_0.2.1       withr_2.4.2           
## [109] broom.mixed_0.2.7      bayestestR_0.11.0      expm_0.999-6          
## [112] parallel_4.1.0         hms_1.1.0              grid_4.1.0            
## [115] rpart_4.1-15           coda_0.19-4            class_7.3-19          
## [118] minqa_1.2.4            rmarkdown_2.10         inum_1.0-4            
## [121] partykit_1.2-15        gvlma_1.0.0.3          shiny_1.7.1

options(warn = defaultW)

Review on Stack Overflow Survey Results

By Horace Lee

21 Nov 2021

Problem Statement

Import

1. DATA PRE-PROCESSING

Top 10 Countries with most Respondents

Identifiy the most common Developer Type, Language & Database

Tidy & Transform

2. DATA EXPLORATION

Remove Exceptionally High Income

Detect and Remove Outliers

Inconsistent Survey Data (Exclude 2021 & Poland/Brazil)

BoxCox Transformation

Normality Test

3. MODELLING ANALYSIS

Part 1A: Income Differentiation by Country (ANOVA)

Post Hoc Analysis (Tukey HSD)

PART 1B: Net Income / Purchasing Power Differentiation by Country

Web Scraping

Adjusting Income with Tax Rate & Big Mac Index

PART 2: Interplay Education & Coding Experience on Income (for US Only)

BoxCox Transformation for US

US Model 1: Interaction with Python Language

US Model 2: Interaction with Education Level

Chart of Estimates

Adjusted for Education Level, controlling fPython & xSex

US Model 3: Interaction with Education Level (Undergrad vs Postgrad)

Adjusted for Education Level, controlling fPython & xSex

Step 1: Plot interaction using interact_plot()

Step 2: Run simple slopes analysis

Step 3: Spotlight analysis (set J-N at T; SWITCH pred and modx)

Step 4: Run interaction_plot() again by adding regions of significance

Specify OLS model

Model Results

Model 1 for US

Model 2 for US

Model 3 for US

Interpretation of the Results

Implications

Limitations and Future Directions

sessionInfo()